don't fire missing act() warnings for react-art (#15975)

* use toWarnDev for dom fixture tests

forks toWarnDev from root into fixture/dom, updates tes tests to use it

* disable act() warnings for react-art()

- For 'secondary' renderers like react-act, we don't want to fire missing act() warnings; the wrapping renderer will fire warnings anyway, and when it flushes, it flushes effects *across* renderers.

- I could have used isPrimaryRenderer as the flag, but this is marked as false for react-test-renderer, and we *do* want the warning to fire for it. Hence a new flag.

* add missing dependency `art` to fixtures/dom

Author: threepointone

src/ReactTestHostConfig.js

@@ -261,6 +261,7 @@ export function createTextInstance(
 }
 
 export const isPrimaryRenderer = false;
+export const shouldWarnUnactedUpdates = true;
 
 export const scheduleTimeout = setTimeout;
 export const cancelTimeout = clearTimeout;